This invention relates to helium cooled nuclear reactors and is mainly concerned with reactors of the kind known as high temperature reactors (HTR).
A helium cooled high temperature reactor may comprise a mass of graphite moderator embodying nuclear fuel and through which the gas coolant is passed in heat exchange with the fuel. The coolant leaves the reactor core at approximately 750.degree. C. A typical closed loop coolant circuit comprises the nuclear reactor core and a boiler unit wherein steam is generated by heat exchange with the coolant gas. The helium circuit of an HTR contains small quantities (for example a few parts per million) of impurities, for example, hydrogen, methane, carbon monoxide and water which, to some extent, arise from inleakage of water and hydrogen from the boiler unit. It is desirable to minimise the level of impurities in the coolant and the impurities are usually kept to tolerable values by circulating a fraction of the gas through a purification by-pass loop. The by-pass loop comprises an oxidising bed and an absorption bed for removing the oxidised species. However, impurities cannot be entirely eliminated and, owing to the fact that the water content of the gas is inherently low (because of the affinity of high temperature graphite for water) the ratio of hydrogen to water is generally high. Such a high ratio can be disadvantageous, by preventing, for example, the formation of oxide films on mild steel components. Although the ratio can be reduced by injection of water, this raises the absolute levels of both hydrogen and carbon monoxide. These side effects are undesirable, the hydrogen for example giving rise to additional methane which can lead to carburisation of the circuit materials. The additional graphite loss arising from water injection is also undesirable.